When a sheet is transported between a nip formed by nip-forming surfaces it is desirable that after passing through the nip the sheet should not remain in contact with one of the nip-forming surfaces but should be and remain separated therefrom. Two different phases can be described in such separation: a first phase in which the leading part of the sheet comes out of the nip, and a second phase in which the remainder of the sheet comes out of the nip. The sheet separation during the first phase, i.e. disengagement of the sheet from the nip-forming surfaces at the exit side of the nip, is governed by a number of factors, such as the nip geometry, the bending strength of the sheet and the forces operative on the sheet at the nip. The leading edge of a sheet will readily disengage from a nip-forming surface if it has a small radius of curvature at the exit side, but will disengage with greater difficulty from a nip-forming surface if it has a large radius of curvature at the exit side. Adhesion forces and electrostatic forces operative in the nip will also render separation difficult. With conventionally formed nips, sheet separation is readily achieved during the first phase for many types of sheets. When however a large portion of the sheet has passed through the nip, that portion, since it is limper, may be pulled back to one of the nip-forming surfaces by a small force, so that the separation achieved in the first phase is destroyed in the subsequent second phase. This may readily occur particularly as a result of electrostatic attraction forces continuing to be operative between the sheet and the nip-forming surface and particularly when a nip-forming surface has a large radius of curvature at the nip exit side.
In the sheet handling device described in U.S. Pat. No. 4,062,631, the nip-forming surfaces are formed by a first roller and by a belt which is pressed against the first roller by a second roller, the belt having a larger radius of curvature at the nip discharge side than the first roller. The guide member consists of a guide roller disposed at the exit side of the nip at a fixed location and rotating in one direction. The guide roller exerts an electrostatic attraction force on a charged sheet coming from the nip which tends to keep the sheet separated from the belt in opposition to the electrostatic attraction which exists between the charged sheet and the belt. Since the guide roller cannot be disposed close to the nip, because of its roller shape, the separation force exerted by the guide roller cannot become operative until a relatively large portion of the sheet has left the nip. In addition, the separation force must overcome the force of attraction between the sheet and the belt. The separation force must be generated by the charge present on the sheet, so that the sheet must be capable of receiving a considerable charge for the purposes of separation. This restricts this device to processing specific types of sheets.
The devices shown in U.S. Pat. Nos. 4,004,802 and 4,370,048 as well as German Patent No. 475,563 all disclose a guide member for guiding a sheet at the exit side of a transport nip away from the nip-forming surfaces wherein an end of the guide member contacts one of the nip-forming surfaces, preferably the surface having the greater radius of curvature. Damage to the nip-forming surface can result from repeated contact by the guide member.
The devices shown in U.S. Pat. Nos. 3,820,776 and 3,506,259 as well as those shown in the IBM Technical Disclosure Bulletins entitled "Mechanical Stripper", Vol. 16, No. 9, February 1974 and "Mechanical Stripper with Drum Stop", Vol. 19, No. 5, October 1976, are for separating the leading edge of a sheet firmly attached to one moving surface such as a photo-conductive drum. These devices do not show a guide device used at the exit end of a nip for guiding a sheet away from the nip-forming surfaces.
It would be desirable therefore, to have a sheet guiding device for use at the exit end of a nip which did not have these disadvantages and which was suitable for 1 processing many different kinds of sheets.